1. Field of the Invention
This invention relates generally to optical data storage systems and more specifically to a data storage system having multiple data surfaces.
2. Description of the Prior Art
Optical data storage systems provide a means for storing great quantities of data on a disk. The data is accessed by focussing a laser beam onto the data layer of the disk and then detecting the reflected light beam. Various kinds of systems are known. In a ROM (Read Only Memory) system, data is permanently embedded as marks in the disk at the time of manufacture of the disk. The data is detected as a change in reflectivity as the laser beam passes over the data marks. A WORM (Write Once Read Many) system allows the user to write data by making permanent marks, such as pits. Once the data is recorded onto the disk it cannot be erased. The data in a WORM system is also detected as a change in reflectivity.
Erasable optical systems are also know. These systems use a laser to heat the data layer above a critical temperature in order to write and erase the data. Magneto-optical recording systems record data by orienting magnetic domains of a spot on the data layer in either an up or down position in order to form a recorded mark. The data is read by directing a low power laser to the data layer. The differences in magnetic domain magnetization direction of the recorded marks on the data layer cause the plane of polarization of the light beam to be rotated one way or the other, clockwise or counter clockwise. This change in orientation of polarization is then detected.
In order to increase the storage capacity of an optical disk, multiple data layer systems have been proposed. An optical disk having two or more data layers may in theory be accessed at different layers by changing the focal position of the lens. Examples of this approach include U.S. Pat. No. 5,202,875 issued Apr. 13, 1993 by Rosen et al; U.S. Pat. No. 5,097,464 issued Mar. 17, 1992 by Nishiuchi et al; and U.S. Pat. No. 4,450,553 issued May 22, 1984 by Holster et al.
One problem with using multiple data surface magneto-optical media is that the magneto-optical material is highly absorptive of light. Multiple data surface media require that the upper data layers allow light to pass through to the lower data layers. The magneto-optical material tends to absorb a high percentage of the light, and so the small amount of light received from the lower data surfaces results in a weak data signal. What is needed is a magneto-optical multiple data surface system which provides a good signal from even the lower data layers, as well as allowing writing and erasing with reasonable laser powers on all data layers.